This invention relates to controls for electric motors and is particularly useful for controlling universal motors of hand held portable electric tools.
Controls of the aforementioned type are commonly incorporated within a self contained switch structure such as that shown in U.S. Pat. No. 3,775,576 issued Nov. 27, 1973 to Harry W. Brown. Switches of this type are housed in the handle of the portable electric tool and have a depressable operator which is controlled by the index finger of the hand holding the tool. Depression of the operator actuates a switch for connecting the motor to a source of electric power and increases the speed of the motor as a function of the amount of depression of the operator. It is frequently desirable that the tool be capable of starting from an OFF condition without the motor causing a torquing or jerking of the tool in the hand of the user. It is further desirable that the speed of the motor be maintained at the selected level when the tool is subjected to a loading condition.
The aforementioned desirable qualities are accomplished by providing soft-start and feedback circuits in the control. However, in consideration of cost and compact packaging of such controls, particular attention need be given to the number, size and cost of the components of the control system. Half-wave controls have been predominantly used, particularly where feedback is incorporated, because the number of components may be significantly reduced. However, half-wave controls limit the speed range of the motor to approximately 85% of full speed. Full-wave controls provide a greater range of motor speed control, but generally require several large power handling components which increase cost and package size of the switch housing the control and create heat dissipation problems for the components. Present speed controls which incorporate soft-start and feedback features utilize sensing resistors, uni-junction transistors, pulse transformers, and thermistors. These components are generally power handling components which are relatively high in cost, physically large, consume significant space, and increase the complexity of the control and its assembly operation. Moreover, significant power is dissipated by these components that must be removed from the control and that is not available to the motor being controlled.